Prime Minister Shinzo Abe stormed to a landslide "super-majority" in snap Japanese elections, near complete projections showed Monday, with the hardline nationalist immediately pledging to "deal firmly" with North Korea.

Abe's conservative coalition is on track to win at least 312 seats with only a handful left to call, according to public broadcaster NHK, giving him a coveted two-thirds majority in the lower house of parliament.

That will allow him to pursue his cherished goal of proposing changes to the country's pacifist constitution to beef up the status of the military, which is effectively restricted to self-defence.

Abe, 63, is now on course to become Japan's longest-serving premier, winning a fresh term at the helm of the world's third-biggest economy and key US regional ally.

The hawkish prime minister said the crushing election victory had hardened his resolve to deal with the crisis in North Korea, which has threatened to "sink" Japan into the sea and fired two missiles over its northern islands.

"As I promised in the election, my imminent task is to firmly deal with North Korea. For that, strong diplomacy is required," stressed Abe, who has courted both US President Donald Trump and Russian President Vladimir Putin.

'Some concerns' about Abe

However, while local media acknowledged what was described as a "landslide" victory, many chalked up Abe's win to a weak and ineffective opposition and urged caution.

"The voters didn't think the opposition parties were capable of running a government... they chose Prime Minister Abe, who is at least better, even if they had some concerns about the ruling coalition," said the Nikkei daily.

The Asahi newspaper said: "The Abe brand is not as strong as it was before. There are some signs that voters are seeking a change in the situation whereby Abe is the only decent option."

"Winning an election in a democracy doesn't give the winner carte-blanche and he would be overconfident if he thought people were satisfied with the past five years of government management," said the paper.

According to an exit poll by Kyodo News on Sunday, 51 percent of voters said they do not trust Abe with 44 percent saying they did.

Turnout was expected to be only a fraction higher than all-time low set in the 2014 election and was boosted largely by people voting early to avoid a typhoon, which smashed into Japan on election day.

The opposition Party of Hope, formed only weeks before the election by the popular Tokyo Governor Yuriko Koike, suffered a drubbing. It won just 49 seats according to the NHK projections.

A chastened Koike, speaking thousands of kilometres away in Paris where she was attending an event in her capacity as leader of the world's biggest city, said it was a "very severe result" for which she took full responsibility.

The new centre-left Constitutional Democratic Party out-performed Koike's new group but still trailed far behind Abe with 54 seats.

"People are reluctant about Prime Minister Abe, but then who would you turn to? There is no one," said Naoto Nonaka, professor at Gakushuin University in Tokyo.

'Deepen constitution debate'

Abe, who has in the past been criticised for an arrogant attitude towards voters, vowed to face the challenge posed by the victory "humbly."

He struck a cautious note on possible revisions to the US-imposed constitution, saying that he would "deepen" debate in parliament on the divisive issue but not seek to ram anything through.

"I don't plan to propose (changes) via the ruling bloc alone. We'll make efforts to gain support from as many people as possible."

Any changes to the document must be ratified by both chambers of parliament and then in a referendum, with surveys showing voters are split on the topic.

Many voters stressed that the economy is their biggest concern, as the prime minister's trademark "Abenomics" strategy of ultra-loose monetary policy and huge government spending has failed to rekindle the former Asian powerhouse.

Abe has vowed to use the proceeds from a planned sales tax hike to fund free childcare in a bid to get more women into the workforce.

"Neither pensions nor wages are getting better... I don't feel the economy is recovering at all," said 67-year-old pensioner Hideki Kawasaki as he cast his vote in a rain-swept Tokyo.

But investors cheered the victory, with the benchmark Tokyo index up 1.15 percent, extending a winning run that has seen 14 straight consecutive gains -- the first since 1961.